Move tenantName out into the auth dict

According to the auth 2.0 docs at
http://docs.rackspace.com/auth/api/v2.0/auth-client-devguide/content/POST_authenticate_v2.0_tokens_Token_Calls.html
the tenantName needs to be directly in the auth node, according to
example 4.41. This change moves it out of the
RAX-KSKEY:apiKeyCredentials node and into the auth node.

Tested with production Rackspace cloud environments.